Question 1

Which experimental result from the Meselson–Stahl experiment provided the strongest evidence for the semiconservative model of DNA replication?

Accepted Answer

After one generation, DNA consisted of both heavy and light nitrogen, forming a single intermediate band.

Question 2

DNA polymerase can only add nucleotides to which end of a growing DNA strand?

Accepted Answer

$$3'$$ end

Question 3

Why is DNA synthesis considered exergonic, even though polymerization should be endergonic?

Accepted Answer

The hydrolysis of the two extra phosphates from each dNTP releases energy that drives the reaction.

Question 4

In eukaryotic cells, why are multiple origins of replication necessary?

Accepted Answer

Eukaryotic chromosomes are much larger and linear, requiring multiple starting points to replicate efficiently.

Question 5

Which enzyme is responsible for relieving the tension created ahead of the replication fork during DNA unwinding?

Accepted Answer

Topoisomerase

Question 6

What is the role of primase during DNA replication?

Accepted Answer

It synthesizes a short RNA primer to provide a $$3'-OH $$group for DNA polymerase.

Question 7

Which of the following best describes the synthesis of the lagging strand during DNA replication?

Accepted Answer

It is synthesized in short fragments away from the replication fork, later joined by DNA ligase.

Question 8

A mutation disables the function of single-strand DNA-binding proteins (SSBPs) in a cell. What is the most likely consequence during DNA replication?

Accepted Answer

The separated DNA strands would re-anneal, preventing replication.

Question 9

Why do linear chromosomes in eukaryotes face the 'end replication problem'?

Accepted Answer

DNA polymerase cannot add nucleotides to the $$5'$$ end after the last RNA primer is removed.

Question 10

What is the function of telomerase in eukaryotic cells?

Accepted Answer

It extends the $$3'$$ end of the lagging strand template using an internal RNA template.

Question 11

Which of the following statements about telomeres is correct?

Accepted Answer

They consist of short, repeating, non-coding DNA sequences at the ends of linear chromosomes.

Question 12

Why do most somatic cells have a limited number of divisions, according to the telomere hypothesis?

Accepted Answer

Their telomeres shorten with each division, eventually triggering cell division to stop.

Question 13

Cancer cells often have active telomerase. What is a likely consequence of this?

Accepted Answer

They can divide indefinitely because their telomeres do not shorten.

Question 14

DNA polymerase has proofreading ability. What is the mechanism by which it corrects errors during DNA synthesis?

Accepted Answer

It uses its exonuclease activity to remove mismatched nucleotides from the $$3'$$ end.

Question 15

Which repair mechanism is responsible for correcting mismatched bases that escape proofreading during DNA replication?

Accepted Answer

Mismatch repair system

Question 16

Exposure to UV light can cause thymine dimers in DNA. What is the main consequence of these dimers if not repaired?

Accepted Answer

They create a kink in the DNA, blocking replication and transcription.

Question 17

How does the nucleotide excision repair system fix DNA damage such as thymine dimers?

Accepted Answer

It removes the damaged single-stranded DNA and uses the undamaged strand as a template for repair.

Question 18

A patient is found to have a genetic disorder that results in frequent mutations due to a failure in the mismatch repair system. Which of the following best explains the molecular basis for this increased mutation rate?

Accepted Answer

Mismatched bases that escape DNA polymerase proofreading are not corrected, leading to permanent mutations.

Question 19

Which of the following best describes the directionality of DNA synthesis?

Accepted Answer

DNA is always synthesized in the $$5' \rightarrow 3'$$ direction.

Question 20

A scientist is studying a new drug that inhibits DNA ligase. Which of the following processes would be most directly affected in a cell treated with this drug?

Accepted Answer

Joining of Okazaki fragments on the lagging strand

DNA and the Gene: Synthesis and Repair – Study Notes

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